Exhaust gas is emitted by engines as a result of fuel combustion. Exhaust gas typically contains nitrogen, water vapor and carbon dioxide, but can also include carbon monoxide and particulate matter. Particulate matter consists either of solid carbonaceous materials (e.g., soot or non-volatile particulates) or condensed volatile matter which is referred to as volatile particulates. Emissions of both volatile and non-volatile particulates are known to increase with high concentrations of the heavier hydrocarbon fractions in the fuel. Engines that burn fossil fuels tend to produce the most particulate emissions during a cold start. In the case of gasoline engines, most of the particulate emissions are released within the first few minutes of a cold engine start. Similar emission levels are observed in jet engines on aircraft. In addition to cold starts, jet engines produce higher levels of emissions during take off.
Membrane distillation has recently become increasingly popular in a variety of fluid-treatment applications. The membranes are typically hydrophobic and microporous to keep the feed solution separated from that of the distillate during operation. Both hollow fiber and flat sheet membranes have been used in laboratory studies. For membrane distillation, hollow fiber membranes are typically employed in tube/shell configurations, where bundles of hollow fiber membranes are arranged along the longitudinal axes of the modules. Each hollow fiber membrane in the bundle is typically a hydrophobic, microporous membrane having an exterior surface and an inner hollow tubular region. In some membrane distillation systems, the inner hollow tubular regions define a tube side of the module, which serves as a conduit to contain the feed solution. The exterior surfaces of the hollow fiber membranes face a shell side of the module, which provides a region for collecting the distillate fluid separated from the feed solution.
During membrane distillation, the feed solution is typically heated to form a temperature differential across the hollow fiber membranes. This temperature differential creates a vapor pressure differential between the tube side and the shell side of the membranes in the module, which causes a portion of the feed solution to evaporate near the pore entrance on the feed solution side and the vapor to transmit through the hollow fiber membranes. The transmitted vapor then condenses at a gas/liquid interface near the pore entrance of the membranes on the distillate side, thereby providing the desired distillate. According to the present invention, membrane distillation concepts can be used to reduce engine emissions.